Tactile sensor element and sensor array

ABSTRACT

A tactile sensor element includes a first pressure transfer layer and a second pressure transfer layer, an elastomeric body arranged between the first and second pressure transfer layers, the body having a first and a second surface opposed to each other, the first and second surfaces having corrugations to allow displacement of elastomeric body material in a predetermined direction perpendicular to the corrugations when exposed to a contact pressure on at least one of the surfaces, a first electrode arranged on the first surface and a second electrode arranged on the second surface, the first and the second electrodes being connectable to external means for determining the capacitance of a capacitor formed by the elastomeric body and the electrodes, where at least one pressure transfer layer has at least one portion of increased thickness. Further disclosed is a tactile sensor array comprising a plurality of sensor elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in international PatentApplication No. PCT/DK2003/000848 filed on Dec. 10, 2003 and DanishPatent Application No. PA 2002 01908 filed on Dec. 12, 2002.

FIELD OF THE INVENTION

The invention relates to a sensor and, more particularly to a tactilesensor element for converting the pressure acting on a surface of asensor into an electrical signal.

BACKGROUND OF THE INVENTION

Tactile sensor and tactile sensor arrays may be used in variousapplications to provide informations on the magnitude and the localdistribution of pressure applied on a given surface area. As examplescould be mentioned touch pads or drawing pads for character recognitionin computer equipment, or contact pads for the determination and controlof grip forces of a robotic gripping tool.

Such sensors widely known are using pressure sensitive devices based onpiezo-resistive, piezo-electric or capacitive effects, and may bemanufactured by thickfilm or thinfilm technologies, or using discreteelements. Miniaturization of such sensors is typically obtained bytaking advantage of silicon micromachining processes. Common to theseknown sensors are relatively high costs for manufacturing and packaging.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a novel tactile sensorelement and a novel tactile sensor array, which are advantageous inregard of sensitivity, miniaturization, stability and manufacturingcost.

A tactile sensor element according to the invention comprises a firstpressure transfer layer and a second pressure transfer layer, anelastomeric body arranged between the first and second pressure transferlayers, the body having a first and a second surface opposed to eachother, the first and second surfaces having corrugations to allowdisplacement of elastomeric body material in a predetermined directionperpendicular to the corrugations when exposed to a contact pressure onat least one of the surfaces. The sensor element further comprises afirst electrode arranged on the first surface and a second electrodearranged on the second surface, the first and the second electrodesbeing connectable to external means for determining the capacitance of acapacitor formed by the elastomeric body and the electrodes.

The corrugation of the surfaces of the elastomeric body with electrodesarranged on these surfaces ensure compliance of the electrodes in caseof stretching the elastic body in a direction perpendicular to thecorrugation. The electrodes can follow the geometric deformation withoutchanging their electrical properties, and without detaching from thebody surfaces or even breaking. Applying pressure to the pressuretransfer layers leads to a decrease in thickness of the elastic body andto a decreased distance between the electrodes. The capacitance of acapacitor formed by the electrodes will thus be increased. Measurementof the capacitance by external equipment will provide information on thepressure acting on the surface of the tactile sensor element.

At least one portion of at least one pressure transfer layer has anincreased thickness in relation to the other portions of the layer.

Hereby, the pressure acting on the tactile sensor element will betransferred to the elastomeric body by squeezing the body only in aregion corresponding to the portion of increased thickness. Onactivation of the sensor, by pressing on the pressure transfer layer,the displaced material of the body is accommodated in the space formedbetween the thinner portions of the pressure transfer layer and theelastomeric body.

In a preferred embodiment, at least one pressure transfer layer has acentral portion of increased thickness and, on each side of the centralportion in the predetermined direction of extension of the body, an endportion of decreased thickness. Hereby, the tactile sensor element isdivided into three sections, where the elastomeric body in a centralsection adjacent to the central portion of the pressure transfer layerwill be squeezed and reduced in thickness. Body material from thecentral section will be pressed in a direction perpendicular to thecorrugations towards the body sections adjacent to the end portions ofthe pressure transfer layer. The thinner transfer layer in theselocations allows an increase in thickness of the body due to the bodymaterial displaced from the central section. However, it is important tonote, that the overall longitudinal dimensions of the sensor element inthe direction of extension will be substantially unchanged.

Preferably, the electrode on at least one of the surfaces comprises afirst electrode portion adjacent to the central portion of the pressuretransfer layer and second electrode portions adjacent to the endportions of the pressure transfer layer, the first and second electrodeportions being isolated from each other. If pressure is applied to thesensor element, the first electrode portions covering the centralsection of the body form a first capacitor with increasing capacitance,while the second electrode portions form a second capacitor withdecreasing capacitance. It is now possible to measure a differentialcapacitive signal as a function of pressure. This is advantageous toincrease the sensor sensitivity and to eliminate measurement errors dueto changing environmental conditions.

Preferably, the surface area of the first electrode portion issubstantially equal to the total surface area of the second electrodeportions. Hereby, the initial capacitance for both capacitors issubstantially equal, which simplifies the determination of differentialcapacitance with the external electronic means.

Preferably, lateral means are provided on two opposed sides of thesensor element to prevent overall dimensional change of the sensorelement in the predetermined direction. Hereby it is assured, that theforce applied to the pressure transfer layer is utilized fordisplacement of the body material, which increases the sensitivity ofthe sensor element.

Preferably, the thickness of the pressure transfer layer issubstantially equal to the thickness of the elastomeric body. Hereby,the body will be subjected to the major part of the pressure forcesacting on the surfaces of pressure transfer layers.

Preferably, the elastomeric body and the pressure transfer layers havesimilar elastomeric properties. Hereby, and in combination with the highcoefficient of friction between the electrodes and the pressure transferlayers, the elastic deformation of both the body and the pressuretransfer layers, resulting from the pressure forces acting on thesurfaces of the pressure transfer layers, can follow each other.

A tactile sensor array is disclosed, comprising a plurality of sensorelements, wherein the sensor elements are arranged in a row and columnconfiguration for the determination of local pressure variations overthe surface area of the sensor array, and wherein the plurality ofsensor elements being integrally formed in a common elastomeric bodymember. A sensor array of this kind shows significant manufacturing costadvantages, as a large amount of sensor element bodies can bemanufactured in a single process. High resolution tactile sensor arrayscan be provided, where each sensor element presents a pixel-like elementof the array.

Preferably, each row of sensor elements comprises an elongated commonelastomeric body member, the body member constituting a continuoussequence of sensor element bodies.

Hereby, a row of sensor elements is provided on the basis of a singleelongated elastomeric member with corrugated surfaces. Several rows arearranged to constitute a two-dimensional sensor array.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained in more detail bymeans of embodiments and with reference to the accompanying drawings, inwhich

FIG. 1 shows a side view of a tactile sensor element with a pressuretransfer layer of uniform thickness in unloaded state,

FIG. 2 shows a side view of the same sensor element in loaded state,

FIG. 3 shows a side view of a tactile sensor element with a pressuretransfer layer having portions of increased and decreased thickness, inunloaded state,

FIG. 4 shows a side view of the sensor element in FIG. 3 in loadedstate,

FIG. 5 shows a top view of a tactile sensor array comprising a pluralityof sensor elements of FIG. 3, and

FIG. 6 shows a side view of the sensor array in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a tactile sensor element 1 with an elastomeric body 2,preferably made of silicon rubber sheet material. The body has upper andlower surfaces 3 and 4, which have corrugations in the form of parallelridges and grooves 5 and 6 running across the width of the body in adirection perpendicular to the drawing plane. The ridges and grooves areshown with square profiles, however, other profiles such as sinusoidalor rectangular may be applicable. The depth b of the grooves istypically in the range of 10 to 30% of the total thickness d of thebody, which typically is 10 to 50 μm. As an example, the body has athickness of 20 μm with corrugation depths of 5 μm.

Metal electrodes 7 and 8 are arranged on both upper and lower surfaces 3and 4 by deposition of a thin uniform metal layer, such as gold, silveror copper, by use of suitable deposition technique. The electrodes areconnected with lead wires 11 to external means 12 for measuring thecapacitance of a capacitor formed by the electrodes. Interconnectionbetween the electrodes and the lead wires is established by suitablebonding, welding or soldering technique. The thickness of the electrodelayer is typically in the range of 20 to 100 nm.

The sensor element further comprises upper and lower pressure transferlayers 9 and 10 opposed to and covering the upper and lower surfaces ofthe body. The pressure transfer layers of this embodiment are made ofelastomeric material having similar elastic properties as the bodymaterial. Typically, the thickness for the pressure transfer layers isin the same range as the thickness of the body. The lower pressuretransfer layer 10 is in contact with a support body 13 to preventrelative movement between the support and the pressure transfer layer.

FIG. 2 shows the sensor element of FIG. 1 exposed to a pressure p on topof the upper pressure transfer layer 9. The pressure transfer layers arenow brought into contact with the surfaces 3 and 4 of the elastomericbody, transferring the pressure to the body, which will be subjected toelastic deformation. Due to the corrugated structure of the bodysurfaces, the electrodes 7 and 8 deposited thereon are compliant in alongitudinal direction perpendicular to the ridges and grooves.Compliance of the electrodes in the direction of ridges and grooves isnegligible due to the high elastic modulus of the electrode material.Thus, squeezing of the body leads to a reduction in thickness and to anelongation of the body in the longitudinal direction. The coefficient offriction between the electrodes and the material of the pressuretransfer layer are sufficiently high to avoid relative movements betweenthe body with the electrodes and the surface of the pressure transferlayers. Due to the similar elastomeric properties and thicknessdimensions of pressure transfer layers 9, 10 and body 2, the elongationof the elastic pressure transfer layers and the body are therebyfollowing each other. External means 12 will now measure an increasedcapacitance, as the distance between the electrodes is reduced. A linearrelationship is achieved between the applied pressure and the measuredcapacitance change.

A tactile sensor element according to the invention is shown in FIG. 3in unloaded state and in FIG. 4 in loaded state. Each of the pressuretransfer layers 9 and 10 now comprise a central portion 20 of increasedthickness and end portions 21 and 22 of decreased thickness.

Between the pressure transfer layers is arranged an elastomeric body 2with corrugated surfaces 3 and 4, having ridges and grooves extendingperpendicular to the predetermined direction of extension of the body 2.The thickness of the central portion 20 of the pressure transfer layers9, 10 is substantially equal to the thickness of the body 2. The firstelectrode deposited on the upper surface 3 of the body is divided into afirst electrode portion 25, arranged adjacent to the central portion 20of the pressure transfer layer 9, and into second electrode portions 26arranged adjacent to the end portions 21 and 22 of the pressure transferlayer 9. In a similar manner, the second electrode deposited on thelower surface 4 of the body is divided into a first electrode section27, arranged adjacent to central portion 20 of pressure transfer layer10, and into second electrode portions 28 adjacent to the end portions21 and 22 of pressure transfer layer 10. The electrode portions arrangedon each surface of the body are isolated from each other, which resultsin three independent capacitors: a first capacitor formed by firstelectrode portions 25 and 27 and two capacitors, each of which is formedby a second electrode portion 26 and a second electrode portion 28.Surface portions 23 of surface 3 and surface portions 24 of surface 4are not covered by electrode material.

The first electrode portions 25 and 27 are connected by lead wires 29(se FIG. 4) to the external means 12 for determining the capacitance ofthe first capacitor. The second electrode portions 26 are connected tothe external means through common lead wire 30, and second electrodeportions 28 through common lead wire 31. Hereby, the capacitance of acommon second capacitor formed by the second electrodes 26 and thesecond electrodes 28 can be measured.

To simplify wiring, all the electrode portions arranged on one surfaceof the elastomeric body could be interconnected to define a commonground electrode.

Second pressure transfer layer 10 is attached to a support body 50.Lateral means 51 for avoiding deformation of the sensor element in thelongitudinal direction are arranged on both sides of the sensor element.

Spaces 52 between the pressure transfer layers and the body areevacuated. Alternatively, pressure equalizing means (not shown) may beprovided between the spaces and the outside environment of the sensorelement.

FIG. 4 shows the sensor element in loaded state, where a load isindicated as a pressure p acting on the upper surface of a relativelysoft pressure transfer layer 9. The elastomeric body 2 is squeezed bythe central portions 20 of the pressure transfer layers. Lateral means51 prevent deformation of the sensor element in the longitudinaldirection. Part of the material of the elastomeric body is beingdisplaced in a direction perpendicular to the ridges and grooves formedin the surface of the body. Accordingly, the thickness of theelastomeric body decreases between the central portions 20, and thethickness of the body increases between the end portions 21 and 22 ofthe pressure transfer layers. The coefficient of friction between theelectrodes and the surface of the pressure transfer layer issufficiently high to avoid sliding movement of the ridges on the surfaceof the pressure transfer layer. Thereby, as it can be seen from FIG. 4,the corrugation pitch, represented by the longitudinal distance betweenadjacent ridges, has increased between the central portions of thepressure transfer layers, and has decreased between the end portions.

The capacitance of the first capacitor formed by the first electrodeportions 25 and 27 has increased, while the capacitance of the capacitorformed by the second electrode portions 26 and 28 has decreased. Adifference in capacitance in dependence of the applied pressure actingon the sensor element can be measured by the external means 12.

FIG. 5 shows a top view of a tactile sensor array 40 comprisingplurality of sensor elements 1 according to FIG. 3 with the firstpressure transfer layer 9 removed.

The array 40 comprises frame portions 41 and several elongatedelastomeric body members 42 in the shape of rubber strips, each bodymember constituting a continuous row of sensor element bodies 2,illustrated by dashed lines. A spacing 43 is provided between adjacentrubber strips or adjacent rows of sensor elements to ensure independentactuation of the sensor elements. Lateral means 51 of the sensor elementin FIG. 3 are also removed. Their lateral limiting function is nowperformed by the neighboring sensor elements within a row of sensors andby the frame 41.

The corrugation of the surface of the body members in the form ofstraight and parallel ridges and grooves extending along the width ofthe strips is only illustrated for a single sensor element body 2.

FIG. 6 shows a side view of the sensor array in FIG. 5, illustrating therealization of a plurality of sensor elements 1 in an elongatedelastomeric body member 42. This is possible by the fact that, uponactuation of a single sensor element, displacement of elastomericmaterial is substantially limited to the region of this sensor element.

Depending on the intended application for a sensor array according tothe invention, the dimensions for a single sensor element (pixel size)may vary from an upper range of several centimeters to a minimum of 100μm. With a length of 200 μm and a width of 100 μm for a single sensorelement 2, the length of the elastomeric body member 42 as shown in FIG.6, which is constituted by six sensor elements, will be 1.2 mm. A squareshape sensor array with a total area of 1.5 mm² would contain twelvebody members, resulting in a number of pixels of 72. It can beunderstood, that high-resolution tactile sensor arrays can be realizedeven with limited overall dimensions.

The scope of the invention shall not be limited to elastomeric bodieshaving straight-line corrugations. Other corrugation shapes asconcentric circles, squares or rectangles may be applicable as well.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent invention.

1. A tactile sensor element comprising: a first pressure transfer layerand a second pressure transfer layer; an elastomeric body arrangedbetween the first and second pressure transfer layers, the body having afirst surface and a second surface opposed to each other, the first andsecond surfaces having corrugations to allow displacement of elastomericbody material in a predetermined direction perpendicular to thecorrugations when exposed to a contact pressure on at least one of thesurfaces; and a first electrode arranged on the first surface and asecond electrode arranged on the second surface; the first and thesecond electrodes being connectable to external means for determiningthe capacitance of a capacitor formed by the elastomeric body and theelectrodes, wherein at least one pressure transfer layer has at leastone portion of increased thickness.
 2. The tactile sensor elementaccording to claim 1, wherein at least one pressure transfer layer has acentral portion of increased thickness and, on each side of the centralportion in the predetermined direction of extension of the body, an endportion of decreased thickness.
 3. The tactile sensor element accordingto claim 1, wherein the electrode on at least one of the surfacescomprise a first electrode portion adjacent to the central portion ofthe pressure transfer layer and second electrode portions adjacent tothe end portions of the pressure transfer layer, the first and secondelectrode portions being isolated from each other.
 4. The tactile sensorelement according to claim 3, wherein the surface area of the firstelectrode portion is substantially equal to the total surface area ofthe second electrode portions.
 5. The tactile-sensor element accordingto claim 1, wherein lateral means are provided on two opposite sides ofthe sensor element for preventing overall dimensional change of thesensor element in the predetermined direction.
 6. The tactile sensorelement according to claim 1, wherein the thickness of the pressuretransfer layer is substantially equal to the thickness of theelastomeric body.
 7. The tactile sensor element according to claim 1,wherein the elastomeric body and the pressure transfer layers havesimilar elastomeric properties.
 8. The tactile sensor array comprising aplurality of sensor elements according to claim 1, wherein the sensorelements are arranged in a row and column configuration for thedetermination of local pressure variations over the surface area of thesensor array, and wherein the plurality of sensor elements beingintegrally formed in a common elastomeric body member.
 9. The tactilesensor array according to claim 8, wherein each row of sensor elementscomprises an elongated common elastomeric body member, the body memberconstituting a continuous sequence of sensor element bodies.
 10. Thetactile sensor array according to claim 9, wherein the elastomeric bodymember has corrugations extending perpendicular to the longitudinaldirection of the elongated body member, and wherein adjacent bodymembers are spaced from each other.
 11. A tactile sensor arraycomprising: a plurality of sensor elements arranged in a row and columnconfiguration for the determination of local pressure variations overthe surface area of the sensor array, and wherein the plurality ofsensor elements being integrally formed in a common elastomeric bodymember; wherein each of the sensor elements comprises a first pressuretransfer layer and a second pressure transfer layer; an elastomeric bodyarranged between the first and second pressure transfer layers, the bodyhaving a first surface and a second surface opposed to each other, thefirst and second surfaces having corrugations to allow displacement ofelastomeric body material in a predetermined direction perpendicular tothe corrugations when exposed to a contact pressure on at least one ofthe surfaces; and a first electrode arranged on the first surface and asecond electrode arranged on the second surface; the first and thesecond electrodes being connectable to external means for determiningthe capacitance of a capacitor formed by the elastomeric body and theelectrodes, wherein at least one pressure transfer layer has at leastone portion of increased thickness.
 12. The tactile sensor arrayaccording to claim 11, wherein each row of sensor elements comprises anelongated common elastomeric body member, the body member constituting acontinuous sequence of sensor element bodies.
 13. The tactile sensorarray according to claim 12, wherein the elastomeric body member hascorrugations extending perpendicular to the longitudinal direction ofthe elongated body member, and wherein adjacent body members are spacedfrom each other.
 14. The tactile sensor element according to claim 1,wherein the at least one portion of increased thickness comes intocontact with the corrugations.
 15. The tactile sensor element accordingto claim 1, wherein the at least one portion of increased thicknesscomes into contact with at least one of the electrodes.
 16. The tactilesensor array according to claim 11, wherein the at least one portion ofincreased thickness comes into contact with the corrugations.
 17. Thetactile sensor array according to claim 11, wherein the at least oneportion of increased thickness comes into contact with at least one ofthe electrodes.
 18. A tactile sensor element comprising: a firstpressure transfer layer and a second pressure transfer layer; anelastomeric body arranged between the first and second pressure transferlayers, the body having a first surface and a second surface opposed toeach other, the first and second surfaces having corrugations to allowdisplacement of elastomeric body material in a predetermined directionperpendicular to the corrugations when exposed to a contact pressure onat least one of the surfaces; and a first electrode arranged on thefirst surface and a second electrode arranged on the second surface;wherein the first and the second electrodes are connectable to externalmeans for determining the capacitance of a capacitor formed by theelastomeric body and the electrodes; wherein at least one pressuretransfer layer has at least one portion of increased thickness; andwherein the electrode on at least one of the surfaces comprise a firstelectrode portion adjacent to the central portion of the pressuretransfer layer and second electrode portions adjacent to the endportions of the pressure transfer layer, the first and second electrodeportions being isolated from each other.